There are several different broadcast standards for television widely in use around the world. Of these, one is the so-called NTSC standard used in the United. States in which television pictures are broadcast at a rate of 30 frames per second, two interleaved fields per frame and 525 lines per frame, and another is the so-called PAL standard used in Europe in which pictures are broadcast at 25 frames per second, two interleaved fields per frame and 625 lines per frame. Movies are captured on film at a rate of 24 frames per second. Top selling television programmes for example “episodics” such as the American television series “ER” are also shot on film at the 24 frames per second rate. Once scenes have been shot the film frames are transferred into an electronic domain by way of a telecine machine.
Plainly, there is an incompatibility between the 24 frames per second of film and the 25 PAL frames and 30 NTSC frames of television. This incompatibility is overcome in one of two ways. If the film is to be shown on NTSC television the film is run through the telecine at 24 frames per second and the film frames are scanned as two television fields. The 24 frames are converted to 30 frames by outputting the first field of the first frame, then the second field and then the first field again. Next, the second field of the second frame is output, then the first. For the third frame the second field is output, then the first and then the second, and for the fourth frame the first field is output first followed by the second. This pattern is repeated for the next group of four frames. The result of this is that 24 film frames (corresponding to one second of moving image) lasts for 60 television fields (also corresponding to one second of moving image). This technique is known as 3:2 pull down and is described for example in our British Patent Application No. 9209204.8 and corresponding U.S. Pat. No. 5,418,572, the teachings of which are incorporated herein by reference.
In both the NTSC and PAL broadcast standards each television frame comprises two interleaved fields. One field comprises all the odd numbered lines in a frame and the other field comprises all the even numbered lines in a frame. If the film is to be viewed on a PAL system at 25 frames per second each film frame is scanned twice (once for each television field) by the telecine. Frames are output from the telecine at a rate of 25 per second. As a result, when the television signals are displayed, one second of film (24 frames) only lasts for 0.96 seconds of television. The speed of action and dynamics in the picture are increased and sound is heard at a slightly higher pitch. Movies and programmes originating on film have always been viewed in this way on PAL televisions.
While these two approaches provide an adequate solution to frame rate incompatibility, they are nevertheless separate operations and it is not possible satisfactorily to convert films or programmes between NTSC and PAL if they have already been converted from the 24 frame movie format to the 25 or 30 frame format of PAL or NTSC. Simply converting from one television standard to the other introduces unacceptable artifacts as the frame rate is changed between 25 and 30 per second and the line rate is changed between 525 and 625 per frame. One way of overcoming this problem is to carry out the editing twice, once to convert from film to NTSC and once to convert from film to PAL. Plainly this is undesirable because it is difficult (dynamics are different at a frame level between the two standards), the effects applied in one standard have to be applied again in the other standard, and costs are increased significantly.
There has recently been a move towards making a single 625 line master tape. The approach that has been adopted is to use a conventional tape-based edit suite with a standard 25/625 VTR adapted so that it is able to record and play the same tape at either the standard 25 frames per second or at a slower 24 frames per second and 625 lines per frame. During the editing process the moving picture is viewed at the correct pace of 24 frames per second and thus the dynamics in the picture are also seen correctly. Once a satisfactory edit has been created and stored on tape, the tape is simply played back at 25 frames per second (4 percent faster than real image time) to give the correct 25/625 rate for PAL.
When an NTSC master tape is required the edited clip is output at 24 frames per second to circuitry which converts the 625 lines to 525 lines and converts the 24 frames per second into 30 frames per second using 3:2 pull down techniques. The resulting video is recorded on to standard 30/525 tape for subsequent playback on standard NTSC equipment. Using this approach also requires the audio to be resampled. The audio signal is normally sampled at a rate of 48 KHz, but when the tape speed is reduced from 25 to 24 frames per second there is a corresponding reduction of the audio sampling rate to 46 KHz. The audio signal is therefore reclocked (without pitch change) from the incorrect 46 KHz sampling rate to the correct 48 KHz sampling rate.
Off-line non-linear editing systems are available which are capable of manipulating 30 frames per second video generated by way of 3:2 pull down. One such system is arranged to remove the additional fields generated by the 3:2 pull down operation and to store only the two fields that make up each frame. The output of such a system is put through an internal 3:2 conversion so that the user of the system views the output at 60 fields per second. The system produces a standard 30 frames per second edit decision list (EDL).
Another approach to overcoming the aforementioned problems is to carry out an edit using such an off-line non-linear editing system in order to make an EDL which is then conformed at either 24 frames per second or 25 frames per second. Either frame rate is acceptable, the important thing is that the edit occurs in the right place. A modified VTR is used to play back video rushes at 24 frames per second to enable 30 frame per second dubs to be produced (via 3:2 pull down). The 30 frame per second dubs are taken into an off-line non-linear editing system, as aforementioned. Editing decisions are made using the system and a standard 30 frames per second EDL is produced. The EDL thus produced is fettled to produce a 25 frames per second EDL which is then used to conform the high-quality 625 rushes in a tape based auto-conform suite to produce the 625 master tape, which is only correctly paced when played at 24 frames per second.
The aforementioned modified VTR may also be used to play back a 25/625 master at 24 frames per second in order to enable the production (via 3:2 pull down) of 30/525 distribution copies from the 25/625 master.
Again, while this approach is able to produce satisfactory results, it is nevertheless laborious, difficult and thus expensive to produce separate 25/625 and 30/525 distribution copies from a single master.